''${currHeader}'' '''

The ${currHeader} macro is given a value whenever
a header-checking rule set is called. Header rule set checking is
declared as part of the H configuration command,
as for example:

LOCAL_RULESETS
HSubject: $>ScreenSubject

Here, sendmail will gather the text following
the Subject: header in the mail message and supply
that text to the ScreenSubject rule set. Usually,
that text is treated as an address. All RFC comments are stripped and
extra interior spaces are removed, but when you want that text to be
supplied intact and as is to a rule set, you can employ this
${currHeader} macro.

To illustrate, consider the need to reject messages that have 10 or
more consecutive spaces in the Subject: header.
Such Subject: headers often indicate a spam
message:

The K line sets up a regular expression that will look for 10
consecutive space characters. The first rule in the rule set attempts
to find 10 consecutive spaces in the workspace by passing the
workspace to the subjspaces regular expression
map.

But this won't work. The workspace contains the
Subject: header's text after that
text has been stripped of RFC comments, and after multiple
consecutive spaces have been reduced to one space. Clearly, the
reduction of spaces will prevent 10 consecutive spaces from being
found. To make this screening work, the first rule needs to be
rewritten like this:

R $* $: $1 $(subjspaces $&{currHeader} $) use this instead

Here, the subjspaces regular expression database
map is instead given the value of the
${currHeader} macro. That value is the
Subject: header's text without
anything removed. All the original spaces are intact, and the spam
message will be successfully rejected.

For another example of a use for this
${currHeader} macro, see Section 25.5.1.